In recent years, progress in devices using organic semiconductors is remarkable, and the studies on a field effect transistor (hereinafter, to be simply called FET) with organic semiconductor thin film have been eagerly conducted. These FET are mostly p-type FET, and few examples have been reported on n-type FET. As a n-type organic semiconductor, fullerene (C60) has been drawing attention. For devices having FET or super lattice structure, two dimensional thin film needs to be manufactured for sequentially forming thin films of different compositions and film thicknesses at atomic level. However, compared with conventional inorganic semiconductors such as silicon and compound semiconductors, their manufacturing level is quite low.
A vacuum deposition method is known as one of the methods of manufacturing these organic semiconductor thin film, and it is known that C60 thin film can be formed two dimensionally on metal. Also, in order to manufacture a field effect transistor from fullerene system materials such as C60, thin films of high quality need to be deposited not on metal but on Si or an insulating substrate. For example, Non-Patent Reference 1 reports forming of C60 thin film on a silicon (Si) substrate terminated with hydrogen.
FIG. 16 is an AFM (atomic force microscope) image of C60 thin film deposited on a Si substrate of Reference 1 (K. Ueno and two others, “Van der Waals Epitaxy on Hydrogen-Terminated Si(III) Surfaces and Investigation of its Growth Mechanism by Atomic Force Microscope”, 1995, Crystal Growth, Vol. 150, pp. 1180-1185), and (A) is an AFM image, and (B) is a diagrammatical cross-sectional view of C60 thin film. Here, the measurement region of the AFM image is 4 μm×4 μm. As is seen from the figure, two dimensional growth is not attained, but columnar crystals of about 10 nm (100 Å) thickness and 1-2 μm size are dotted about on a Si substrate to grow so-called three dimensionally.
The field effect mobility of organic FET using pentacene has a value of about 1 cm2·V−1·s−1. (See, for example, Reference 2: Y. —Y. Lin and three others, Stacked Oentacene Layer Organic Thin-Film Transistors with Improved Characteristics”, 1997, IEEE Electr. Device Lett., Vol. 18, No. 12, pp. 606-608.) This value is larger than that of amorphous Si. In case of organic FET, like the FET of semiconductor, materials of high field effect mobility are desired for increasing electric current driving force.
Recently, References 3 and 4 listed below report that the field effect mobility of a single crystal rubrene along a and b axes has quite large value of 4.4 cm2·V−1·s−1 and 15.4 cm2·V−1·s−1, respectively.
Reference 3: V. C. Sundar and 7 others, “Elastometric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals”, 2004, Science, vol. 303, pp. 1644-1646
Reference 4: R. W. I. De Boer and three others, “Organic Single-Crystal Field-Effect Transistors”, 2004, Phys. Stat. Sol. A, Vol. 201, No. 6, pp. 1302-1331
However, by depositing rubrene on the ordinarily used quartz, corning glass, or sapphire substrate by a molecular beam vapor deposition apparatus, nothing but only amorphous thin film is obtained. FIG. 17 is an image of an atomic force microscope of rubrene thin film of 5×5 μm2 deposited on a conventional sapphire substrate, and the substrate temperatures are (A) room temperature and (B) 100° C. As is obvious from the figure, the obtained rubrene film was amorphous.
For a practical use of field effect transistors of fullerene system materials such as C60, forming of flat two dimensional growth film not on metal but on Si or an insulating substrate is strongly demanded. However, in case to grow C60 thin film on an insulating substrate, it tends to be columnar growth, and thin film of high quality has not so far been obtained.
Also, from conventional rubrene thin film, only amorphous film is obtained which shows almost no function of FET property or others. Further, since this amorphous thin film is quite reactive with oxygen, it becomes transparent when taken out to atmosphere from orange color in vacuum. Thus, thin film forming is difficult, and though quite high field effect mobility is reported for a single crystal, the thin film of high quality needed for practical use of FET has not been obtained.